Evolution__3rd_Edition

12 PART 1 / Introduction
Time
(a) (b)
Fish
Amphibians
Homo
Mammals
Birds
Evolution was accepted, but often
confused with progressive change
Natural selection was widely
rejected ...
Time
Homo
Mammals
Birds
Amphibians
Fish
Figure 1.6
(a) Darwin’s theory suggests that evolution has proceeded as a
branching tree; note that it is arbitrary where Homo is positioned
across the top of the diagram. Homo is often placed at the extreme
right, but does not have to be. The tree should be contrasted
with the popular idea (b) that evolution is a one-dimensional
progressive ascent of life. Darwinian evolution is more like a tree
than a ladder (cf. Figure 1.2).
of evolutionary relationships between animal groups. The famous German biologist
Ernst Haeckel (1834–1919) vigorously investigated the same problem, as he applied his
“biogenetic law” a the theory of recapitulation (which we shall meet in Section 20.2,
p. 573) a to reveal phylogenetic pedigrees.
Although some kind of evolution was widely accepted among biologists, probably
few of those biologists shared Darwin’s own idea of it. In Darwin’s theory, evolution is
not inherently or automatically progressive. The local conditions at each stage mainly
determine how a species evolves. The species does not have an inherent tendency
to rise to a higher form. If Darwinian evolution does proceed in a progressive way,
in some sense, then that is just how things turned out. Most evolutionists of the late
nineteenth and early twentieth centuries had a different conception of evolution from
this. They imagined evolution instead as one-dimensional and progressive. They often
concerned themselves with thinking up mechanisms to explain why evolution should
have an unfolding, predictable, progressive pattern (Figure 1.6).
While evolution a of a sort a was being accepted, natural selection was just as surely
being rejected. People disliked the theory of natural selection for many reasons. This
first chapter is not going to explain the arguments in any depth. What follows here is
only an introduction to the history of the ideas that we shall consider in more detail in
later chapters.
One of the more sophisticated objections to Darwin’s theory was that it lacked a satisfactory
theory of heredity. There were various theories of inheritance at that time, and
all of them are now known to be wrong. Darwin preferred a “blending” theory of inheritance,
in which the offspring blend their parental attributes; for example, if a red male
mated with a white female, and inheritance “blended,” the offspring would be pink.
One of the deepest hitting criticisms of the theory of natural selection pointed out that
it could hardly operate at all if heredity blended (Section 2.9, p. 37).
At a more popular level, many objections were raised against natural selection. One
was that natural selection explains evolution by chance. This was (and still is) a misunderstanding
of natural selection, which is a non-random process. Almost every
chapter in this book after Chapter 4 illustrates how natural selection is non-random,
but the topic is particularly discussed in Chapters 4 and 10. Chapters 6–7 discuss an
evolutionary process called random drift. Random drift is random, but it is a completely
different process from natural selection.
..